[docs] updated the part about Boost in installation guide

[StrikerRUS]

• Add a note about custom CUDA paths in Windows guide, like in Linux one.
• Encourage users to pass Boost paths via CMake options instead of environment variables to deal with CMP0074. I tested on Windows 10 with Visual Studio 2017 the following cases: CMake 3.14.1 and Boost 1.69.0 (after adding CMP0074); CMake 3.11.0 and Boost 1.65.1 (before adding CMP0074). They worked fine.

Closed #2081.
Refer to

• Error: Please install CMake first #2081 (comment),
• Cuda GPU install bug , windows10 #1632,
• https://cmake.org/cmake/help/v3.14/policy/CMP0074.html,
• https://cmake.org/cmake/help/v3.14/module/FindBoost.html.

[StrikerRUS]

@huanzhang12
Another case of successful installation by this guide: #2096.

[StrikerRUS]

@huanzhang12 Please review.